As a guess.. I'd say the small size posed problems with cable run, the straddle carrier fouling the frame inside the third triangle when lever is pulled.
Thingeo you know whatjamacallit.
Found four other bikes that way all 14.5"
Plus the serial shows it production not proto.
But still just four, and yours is probably one one of them rebuilt.
Still a lovely frame....and definitely rare nonetheless
...I think it would have had 2X4 forks...but I'm sure someone's going to snap her up. Good luck with the sale.